In the field of optical communication, optical switches that perform switching of light according to a voltage that causes the refractive index of a crystal having an electro-optical effect (electro-optical crystal) to change are known.
Among those, waveguide type optical switches such as a directional coupling optical switch using the proximity effect of two waveguides, and a Mach-Zehnder interferometer-type photonic switch that generates a phase difference between lights that propagate the waveguides according to an external voltage applied between waveguides, and that uses optical interference that occurs therebetween have been proposed. Since these waveguide type optical switches can change the refractive index at high speed, they can perform switching at high speed.
As another type, an optical switch that uses the Bragg effect presented for example in Japanese Patent No. 2666805 (hereinafter referred to as Patent Document 1) is known.
FIG. 1 is a perspective view showing a structure of an optical switch according to a related art reference.
As shown in FIG. 1, the optical switch according to the related art reference has optical waveguide layer 2 made of a non-linear optical substance having the electro-optical effect (electro-optical crystal); and first electrode group 11 and second electrode group 12 that are formed in optical waveguide layer 2.
First electrode group 11 and second electrode group 12 each are composed of a plurality of planar electrodes 1 that can expand and make contact in the direction of the thickness of optical waveguide layer 2. The individual electrodes of first electrode group 11 and those of second electrode group 12 are alternately arranged at a predetermined interval such that the cross-section of a plane of the electrodes perpendicular to the direction of the thickness of optical waveguide layer 2 is formed in the shape of a comb.
When a voltage is applied between first electrode group 11 and second electrode group 12 of the optical switch shown in FIG. 1, the refractive index of the nonlinear optical substance of optical waveguide layer 2 periodically changes. The portion of the nonlinear optical substance in which the refractive index periodically changes functions as a diffraction grating that reflects incident light, so called the Bragg reflection. On the other hand, when the voltage applied between the first and second electrode groups is stopped, since the portion does not function as a diffraction grating, the incident light passes through the region between the planar electrodes.
When the foregoing optical switch is used for optical communication or the like, the extinction ratio that represents the difference between the intensity of transmitted light in the ON state and that in the OFF state can be around 10:1. However, when the optical switch is used for example for an optical modulator of an image display device, an optical switch having a higher extinction ratio than is used for optical communication or the like is desired so as to improve the luminance and contrast ratio.
In addition, an optical switch used for an image display device or the like needs to have a high optical damage resistance. An optical switch used for an image display device needs to modulate light of several ten to several hundred mW or greater. The size of the waveguide used in the foregoing waveguide type optical switch is typically several μm. Since the intensity of light per unit square with which such a waveguide type optical switch is irradiated is high and thereby the nonlinear optical crystal or the like tends to be optically damaged, it is difficult to use such an optical switch for an image display device.
In addition, when the refractive index of the electro-optical crystal is caused to be changed according to an electric field applied thereto, the refractive index changes depending on the temperature of the crystal. When the magnitude for which the refractive index changes fluctuates according to the temperature, the intensity of output light of the optical switch also changes. Thus, to stably operate the optical switch, the temperature at which the refractive index changes in the electro-optical crystal needs to be maintained in an appropriate range.
The optical switch presented in Patent Document 1 does not have a structure that takes into account how the refractive index of the electro-optical crystal changes depending on the temperature. Thus, when the first and second electrode groups are irradiated with light and thereby their temperatures rise, since the refractive index based on the applied voltage also changes, the wavelength and direction of light reflected by the diffraction grating may become unstable, namely, the operation of the optical switch may become unstable depending on the environment and surrounding temperature of the optical switch.